In the known methods for manufacturing coating masses for road constructions the different grain sizes which are required for manufacturing the required grain mixture of each coating mass are predosed by dosing means and are then commonly subjected to a drying operation. The drying is effected in a known drying drum, which can be operated in a one-direction current mode or a counter-current mode and which comprises an energy source in the form of an oil or gas burner. During this drying operation, a fine grain size fraction is taken off together with the heating gases, which form the heat carrier supplied to the material to be dried. The fine grain size fraction is reobtained in a one-stage or multi-stage filtering means.
The fine grain size fraction taken off together with the heating gases has a relatively heterogeneous grain composition. The grain size stepping depends on several factors, such as the initial humidity, the velocity of the air in the drying drum, the specific weight, the shape of the insertions, the heating medium used, and the like. Also, during the drying operation the grain size composition of the fine grain size fraction entrained by the heating gases varies, especially because of power variations and because of the different grain composition for different prescriptions.
In the known methods for manufacturing coating masses for road constructions, the stone minerals mixed from different grain size ranges are also screened in their hot condition after the drying operation and before being mixed with bitumen. This is necessary in order to assure a highly homogeneous mixture of the stone minerals. The screen apertures and surfaces required for the screening operation are especially remarkable in the grain size range from 0 - 2 mm. Thus, in most cases greater screen apertures are used in order to decrease the screen surfaces and to be able to screen a greater volume per screen surface unit. However, this has the disadvantage that stone fractions in the lower grain size ranges are rather inaccurate. On the other hand, these stone fractions substantially determine the cavity fraction of the finished coating mass. The cavity fraction of a coating, in turn, is significant because of a number of essential qualities such as the rigidness of the bitumen binding, the bitumen fraction, and the compression strength of the coating mass. Known manufacturing methods have the further disadvantage that the period of dwell of the individual grain size fractions in the drying drum varies markedly. In the counter-current mode the fine grain size fraction remains in the drying drum for a relatively long time as it is obtained from the heating gases in certain grain size ranges. However, in the one-direction mode the fine grain size fraction is especially accelerated by the heating gases.
Another essential disadvantage of known manufacturing methods is based on the fact that the fine grain fractions taken off together with the heating gases can to a great extent be considered as a fraction of the grain size range of 0 - 3 mm and only to a smaller extent as a self filler fraction in the grain size range of 0.09 - 0.3 mm. Thus, it becomes obvious that for a given prescription the fractions of smaller grain size ranges are especially difficult to control. However, the essential characteristics of the road coating mass are determined by these fractions.